The present application relates to a seat belt tension sensor which is installed on an anchor section fixing a seat belt to a vehicle body and measures the tension applied to the belt. The application also relates to a seat belt tension sensor which permits easy manufacture, downsizing and accurate detection of a tension. An automobile has an air bag in many cases for ensuring safety for the driver and passengers. An ordinary air bag is designed to have a spread gas pressure sufficient to restrain an adult upon collision at a high speed. When a passenger is a child sitting in a child seat, however, it is not necessary that the air bag inflates in the same manner as to an adult. Recently, therefore, for the purpose of ensuring more appropriate operation of the air bag, there is a tendency toward detecting the condition of the passenger by measuring the seat belt tension and controlling the air bag operation in response thereto.
As an apparatus for detecting the seat belt tension, there is developed an apparatus for calculating the seat belt tension by a microprocessor on the basis of a voltage signal detected by a Hall effect sensor. When this apparatus detects a high belt tension (a belt tension so high as offensive to an ordinary passenger), the air bag control system determines that the child seat is secured by the seat belt, and inhibits the air bag from inflating.
Such a seat belt tension sensor is installed in many cases in an anchor section which fixes an end of the seat belt to the vehicle. Since the anchor section is provided on the seat side of the vehicle body, it is highly probable that moisture or much splashes on the seat belt tension sensor. The seat belt meter of this kind should therefore have sufficient waterproofing property and dusttightness.